The pursuit of low-cost, grid scale battery storage systems has been spurred by the inherent intermittency issues related to renewable energy generation, i.e. solar and wind, and their increasing integration into the power grid. Interest in these systems has also stemmed from their potential to stabilize the grid by balancing peak load during times of high energy demand. The use of redox-flow batteries (RFB) has gained increased attention for these applications. The most attractive feature of these systems relative to traditional lithium ion batteries is their ready scalability: within a RFB, electrolyte materials are dissolved in solution, maintained in a storage vessel, and pumped to the cell compartments.
A RFB stores electrical energy in reduced and oxidized species dissolved in two separate electrolyte solutions, the anolyte and the catholyte. The anolyte and the catholyte circulate through a cell electrode separated by a membrane or separator. Redox flow batteries are advantageous for energy storage because they are capable of tolerating fluctuating power supplies, repetitive charge/discharge cycles at maximum rates, overcharging, overdischarging, and/or because cycling can be initiated at any state of charge.
While the most widely studied and current state-of-the-art systems are based on vanadium redox-flow batteries, the high cost of vanadium has prevented their wide scale distribution. The use of low-cost, water-soluble, organic charge carriers has emerged as a promising alternative to vanadium-based systems. Existing aqueous systems suffer from poor solubility of the charge carriers, side reactions, low cell voltages, instability, and/or capacity fade.